(a) Field of the Invention
The present invention relates to an organic EL (electroluminescence) display device having an improved image quality and, more particularly, to a drive circuit for driving organic EL elements in an active matrix EL display device.
(b) Description of a Related Art
Flat-panel display devices now attract public attention due to their small thicknesses. Among other flat-panel display devices, an organic EL display device has an advantage of low power dissipation. In an EL display device, a plurality of EL pixels are arranged on a substrate in a matrix, each of the EL pixels having one or more of organic thin-film EL element. As a first generation of the EL display device, a simple matrix EL display device using a simple matrix driving scheme is now under development.
The simple matrix EL display device have "m" rows and "n" columns (m.times.n) for pixel elements, wherein each column is supplied with image data and each row is supplied with a scanning signal. An image is displayed on the screen by scanning the "m" rows periodically and sequentially with a constant cycle while supplying the "n " columns with image data.
The simple matrix EL display device has a problem in that a larger dimension of a desired screen reduces the time length used for scanning each row of the EL elements, which causes a reduction of a mean luminance on the screen or an increase of power dissipation for a higher luminance.
Thus, a next generation EL display device using an active matrix driving scheme is expected to solve the above problem.
Patent Publication JP-A-9-305 139, for example, proposes an active matrix organic EL display device such as shown in FIG. 1. The display device includes a plurality of EL pixels P11 to Pmn arranged in a m.times.n matrix. An analog video signal Vs is amplified in a video amplifier, corrected with respect to the characteristics thereof in a V(voltage)/I(current) correction circuit, and then supplied to each of the EL pixels P11 to Pmn. The video signal Vs is supplied to the EL pixels P11 to Pmn intermittently in a time-division system by using a scanning control circuit, which receives a synchronizing signal and controls the timing for the scanning based on the synchronizing signal.
FIG. 2 shows one of the drive units for the EL pixels shown in FIG. 1. Each pixel has an organic EL element 92 and a drive unit 91 for driving the EL element 92. The drive unit 91 includes a transfer transistor 97 controlled by a control signal Cs for receiving the video signal Vs, a storage capacitor 96 for storing the video signal in a frame period until the video signal Vs for the next frame period is supplied, and a drive transistor 95 for driving a corresponding EL element 92 with a current corresponding to the video signal Vs stored in the storage capacitor 96 in each frame period.
When the video signal Vs is to be supplied to a pixel, the transfer transistor 97 is turned on to apply the video signal Vs to a storage capacitor 96 and the gate of the drive transistor 95. The drain current of the drive transistor 95 is supplied to the organic EL element 92 as a cathode current thereof, thereby making the EL element 92 luminous during the frame period based on the drain current of the drive transistor 95.
Each organic EL element 92 in each of the pixels P11 to Pmn has a luminance based on the current supplied by the drive transistor 95, whereby the luminance of the EL element 92 is controlled at a continuous gray-scale level based on the analogue video signal Vs.
FIG. 3 shows a timing chart of the drive unit 91. When the transfer transistor 97 is ON due to an active level of the control signal Cs, the video signal Vs supplied through the signal line 98 is stored in the storage capacitor 96 for a single frame period and turns on the drive transistor 95, which supplies a drive current I.sub.EL to the organic EL element 92 for luminescence based on the gate voltage stored by the storage capacitor 96.
In the organic EL display device as described above, the luminescence of the organic EL element 92 during a single frame period is determined based on the video signal Vs received by the transfer transistor 97. If a dark image succeeds a bright image based on the video signal at the changeover of the frame, as shown in FIG. 3, the potential on the signal line 98 which has changed from a high voltage for a frame period to a low voltage for the next frame period is abruptly applied to the storage capacitor. At this stage, the charge stored in the storage capacitor 96 returns toward the signal line 98 through the transfer transistor 97, which received the next active level of the control signal Vs. In the changeover of the frame period, the gate voltage of the drive transistor 95 is affected by the gate voltage thereof during the precedent frame period, whereby the drive transistor 95 supplies a large current to the organic EL element 92 during the initial stage of the next frame period, thereby raising the luminance thereof above. the desired level, as shown in FIG. 3. This causes malfunction of the EL display device such as a deteriorated image or a poor contrast on the screen.
Patent Publication JP-A-4-247491 describes a drive circuit, which superimposes a blanking signal onto the scanning lines in an active matrix EL display device. In the described drive circuit, however, the blanking signal is supplied during each horizontal scanning period. Thus, this configuration does not solve the above problem caused by the function of the active matrix drive circuit in each frame period or a vertical scanning period.